Galactic Stellar Halo Luminosity Function

Abstract

We measure the luminosity function (LF) of the Milky Way's stellar halo, using a magnitude complete, distance limited sample of stars from $Gaia$ DR3. Stars with high transverse velocities are selected, to isolate a high purity sample of the local halo. We adopt a cutoff transverse velocity of 250$\,$km$\,$s$^{-1}$, yielding 24,471 stars, and compute the halo LF, taking into account the effects of sample selection criteria. The LF displays similar features as are found in the well-probed LF of nearby, metal-rich disk stars, showing a strong peak at an absolute magnitude of around $M_G=10$, and a flattening near $M_G\sim7$ (Wielen dip). The $Gaia$ sample yields the first measurement of the LF continuously from the dimmest main sequence halo stars (subdwarfs) at an absolute $M_G$ magnitude near 13 mag to bright giants at $M_G\sim-3$. We obtain a local stellar halo number density of $1.7\times10^{-4}$ stars$\,$pc$^{-3}$ and disk-to-halo ratio by stellar number density of 480:1. We convert the $Gaia$ $G$ band measurements for our sample stars to Johnson-Kron-Cousins $V$ band, compute the $V$-band halo LF, and compare it to previous studies published over many decades that cover a wide range of techniques used. We discuss applications of the LF to the measurement of the luminosity and stellar mass of the Milky Way halo.

Figures (8)

• Figure 1: Color-magnitude (Hertzsprung-Russell) diagram in Gaia passbands absolute magnitude $M_G$ and color BP$-$RP. The viridis-like-colored scatter-density plot represents the halo stars with $V_\mathrm{t}>250$ km s$^{-1}$, $1/\varpi<1$ kpc, $|b|>36^\circ$, and $G<17$. The underlying magenta scatter-density plot maps a representative disk sample with $V_\mathrm{t}<40$ km s$^{-1}$ and $1/\varpi<0.1$ kpc. Colorbars indicate the number counts.
• Figure 2: Uppermost panel: Number of halo stars in bins of absolute magnitude $M_G$ (black markers) and $M_V$ (red markers) in our final sample. Panel second from the top: LF, that is, the local number density of stars per absolute magnitude ($M_G$ are black markers and $M_V$ red markers) for the stellar halo (excluding white dwarfs) in the solar neighborhood at $Z=0$ pc. All panels: The bin widths are 0.333 magnitudes in absolute Gaia$M_G$ magnitude and absolute Johnson-Kron-Cousins $M_V$ magnitude. Confidence intervals are Poisson uncertainties Gehrels1986. Second from the bottom and lowermost panels: Same as the upper two panels; but the vertical axes are scaled linearly instead of logarithmically, and the horizontal cyan line denotes zero.
• Figure 3: Uppermost panel: Number of halo stars in bins of absolute magnitude $M_G$ (black markers) and $M_V$ (red markers) in our final sample. Panel second from the top: Luminosity density function, that is, the local luminosity density of stars measured in solar luminosities L$_\odot$ per cubic parsec per magnitude $M_G$ (black markers) and $M_V$ (red markers) for the stellar halo (excluding white dwarfs) in the solar neighborhood at $Z=0$ pc. Both panels: The bin widths are 0.333 magnitudes in absolute Gaia$M_G$ magnitude and absolute Johnson-Kron-Cousins $M_V$ magnitude. Confidence intervals are Poisson uncertainties Gehrels1986. Second from the bottom and lowermost panels: Same as the upper two panels; but the vertical axes are scaled linearly instead of logarithmically, and the horizontal cyan line denotes zero.
• Figure 4: Luminosity function for the stellar halo (dot markers) within the solar neighborhood compared with the disk luminosity function GaiaCollaborationSmart2021. Main sequence dwarfs are marked in black, giants in red. The GC-Smart$+$21 disk LF extends to $M_G>20$, but here we plot only till the dimmest extent of the halo subdwarfs ($M_G<15$). The bin widths are 0.333 (halo) and 0.25 (disk) magnitudes in absolute Gaia$M_G$ magnitude.
• Figure 5: Johnson-Kron-Cousins $M_V$ LF for Gaia halo stars within the solar neighborhood compared with stellar halo LFs from the literature. Gaia halo LF uses bin widths of 0.333 magnitude in $M_V$ and the confidence intervals are Poisson uncertainties Gehrels1986. See text and Table \ref{['table:lit']} for details.